1. Technical Field
The field relates to acoustic phase plugs.
2. Description of the Problem
Horn loading a diaphragm type loudspeaker has long been done in order to control the direction of radiation of sound produced. A horn also increases loudspeaker efficiency in air as it operates as a transformer to improve impedance matching between the loudspeaker and the transmission medium. In effect, a horn makes the air adjacent the loudspeaker diaphragm “stiffer.” It was also early recognized that horns progressively cut-off the throughput of middle and high frequency sound with increasing frequency and, consequentially, operate to distort voices and music. Wente in U.S. Pat. No. 2,037,187 noted that the cutoff characteristic was traceable in a large measure to the fact that the sound waves emanating from the various portions of the diaphragm of the loudspeaker traverse paths of unequal length before reaching the throat of the horn. As a result sound waves propagated from different areas of the diaphragm reach the throat out of phase. For low frequency waves the disparity in phase is not particularly detrimental but at progressively higher frequencies the phase difference can increase to a sufficient degree to cause a marked neutralization (or cancellation) of the sound waves with high frequency speech and music being noticeably diminished in intensity. Horn performance was improved by making the sound front introduced to the throat of the horn increasingly “planar.”
Wente proposed a “sound translating device,” or what today would be called a phase plug, to control path distance between sections of a loudspeaker diaphragm and a horn throat. The phase plug was constructed using a tapered cone and a plurality of progressively larger, hollow tapered members with the tapered cone nested in the smallest hollow tapered member and the smallest hollow tapered member nested in the next smallest member to produce a series of annular air ducts/sound channels intermediate a dome shaped portion of the diaphragm and the horn throat.
Henricksen in U.S. Pat. No. 4,050,541 proposed a phase plug having radial rather than annular sound channels. This was done in part to simplify manufacture of the phase plug. Avera in U.S. Pat. No. 6,064,745 taught a phase plug with radial channels where the phase plug had a frusto-conical body with a spherical entrance face which conformed to a loudspeaker diaphragm and a planar outlet face. The channels through the plug exhibited equal or slowly increasing aperture area from entrance face to outlet face to minimize diffraction.
Whether employed with a horn/waveguide or not, generation of a sound field characterized by planar wavefronts from a diaphragm type loudspeaker reduces distortion at a point of reception, particularly at high frequencies. A wavefront is usually defined as a surface of constant phase. The further characterization of a wavefront as being “planar” means that the sound field exhibits constant phase in a flat surface where the surface is perpendicular to the direction of sound propagation. A sound field can consist of successive planar wavefronts.